Treatment for chromium plated aluminum



United States Patent TREATMENT FOR CHROMIUM PLATED ALUMINUM Fred Keller and Walter G. Zelley, New Kensington, la.,

assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application April 9, 1954, Serial No. 422,249

2 Claims. (Cl. 204-140) This invention relates to a treatment for chromium plated aluminum. In particular, it relates to methods of improving the resistance to corrosion of aluminum articles having porous chromium electroplates, As used herein, the term aluminum includes both aluminum of various grades and aluminum base alloys.

Chromium can be applied to aluminum by various electroplating methods, some of which produce fairly thick, impervious chromium layers. For many applications, however, where decorative chromium electroplates are desired, an inexpensive chromium electroplate may be produced which is rather porous and not best suited for resistance to corrosion.

An object of this invention is the provision of a treatment for aluminum articles having porous chromium electroplates which treatment improves the resistance to corrosion of the articles. A further object is the provision of such a treatment which is short, uncomplicated and inexpensive.

We have found that an aluminum article having a porous chromium electroplate may be improved, in the sense of increasing its resistance to corrosion, when it is given a supplemental after-treatment of the character hereinafter described. The after-treatment comprises making the article anode, at a potential of at least 5 volts in an aqueous solution containing up to 1 gram per liter of chromic acid (CrOa), for a period of at least of a minute. This treatment may be effected at room temperature or at elevated temperatures.

By the phrase making the article anode, used in the preceding paragraph, we mean to include making the article periodically anodic (as would be the case if an alternating current source would be employed). Nevertheless, we prefer to use a direct current so that the article remains anodic throughout the treatment. The potential employed is preferably higher than 5 volts, and we usually prefer to employ potentials between 20 and 80 volts, about 40 volts being very satisfactory. The lower potentials are somewhat less effective, and the higher potentials produce somewhat more current flow with a consequent tendency toward deplating of the chromium. However, we have found that even higher potentials may be successfully employed, so that even an ordinary alternating current power supply may be used.

The chromic acid content of the solution may be quite small indeed, since it is merely necessary to have enough chromic acid present to carry a current on the order of l milliampere per square decimeter of anode surface at the voltage employed. The chromic acid content should not, however, exceed about 1 gram per liter of solution. This maximum content is determined by the necessity of avoiding any appreciable dissolution or deplating of the chromium. We have found that a chromic acid content of 0.01 gram per liter of solution is generally adequate, and it is desirable to keep the chromic acid ion content between 0.01 and 0.1 gram per liter of solution. We prefer to employ solutions made up of 0.01 to 0.1

ice

gram of chromic acid anhydride per liter of solution, but we point out that the chromic acid content may also be provided by using solutions of equivalent amounts of soluble chromate or dichromates. Typical soluble chromates and dichromates to which we refer to are the ammonium and alkali metal chromates and dichromates.

The above described treatment requires at least onequarter of a minute to be noticeably effective, but it is not of advantage to extend the treatment beyond about 5 minutes. We have found it most satisfactory to employ treatment times between 1 and 3 minutes. The treatment may be carried out at room temperature (or even at lower temperatures), but we have found that it is most effective when the solution is maintained at a temperature between 65 and 100 C. As a general recommendation, we recommend a two minute treatment at C.

Inexpensive bright chromium electroplates, which are porous, are commercially produced on aluminum by applying a five to fifteen minute deposit from a low temperature solution (18 to 25 C.) directly over a zinc immersion layer. The aluminum surface is usually solvent and alkaline cleaned, rinsed in nitric acid and immersed for about /2 minute in a zincate solution. The aluminum surface having the zinc immersion layer is then given the chromium electroplate just described by plating in a bath of chromic and sulfuric acids employing a current density of 16 amperes per square decimeter. When the electroplate is so produced, in the temperature range mentioned above, a slate gray, porous electroplate (known as cold chrome plate) is produced, but the gray plate may be buffed to an attractive metallic luster. Our invention is particularly applicable to the treatment of aluminum articles having porous chromium electroplates produced by this method, but is not limited thereto. Reference is made to Modern Electroplating, The Electrochemical Society, Inc., 1953, pages 511 to 524 and page 140, for descriptions of conventional and modified zinc immersion procedures and typical cold chromium plating procedures.

We have employed the method of the invention under a wide variety of operating conditions, of which the following are a few examples. We have employed the method on both gray chromium plated aluminum articles and articles on which the chromium plate has been buffed to a bright metallic luster and solvent cleaned. Such articles were made anode at 40 volts in a solution of 0.1 gram per liter of chromic acid for two minutes at 80 C. We have treated similar articles at 40 volts in a solution of 0.01 gram per liter of ammonium chromate for 1 minute at room temperature. We have also treated a number of such articles at various potentials from 5 to volts in solutions of 0.01 gram per liter of chromic acid for two minutes at room temperature. In all cases the articles treated were evaluated against similar articles not given any aftertreatment, and we have noted that the after-treatment improves the resistance to corrosion of the aluminum articles. The resistance to corrosion may be evaluated in a humid atmosphere, in a salt spray, in atmospheric exposure tests, or by other methods.

We claim:

1. A method of improving the resistance to corrosion of an aluminum article having a porous chromium electroplate comprising making the article anode, at a potential of at least 5 volts, in an aqueous solution consisting essentially of water and up to 1 gram per liter of chromic acid, for a period of at least 4 of a minute.

2. A method of improving the resistance to corrosion of an aluminum article having a porous chromium electroplate comprising making the article anode, at a potential between 20 and 80 volts, in an aqueous solution consisting essentially of water and between 0.01 and 0.1

gram per liter of chromic acid, for a period between OTHER REFERENCES and 5 minutes, at a temperature between 65 and 100 C. Anodic Oxidation of Aluminum, pubnshfid in London in 1926 by the Dept. of Scientific and Industrial References Cited in the file of this patent Research, page 6- UNITED STATES PATENTS 5 1,946,151 Edwards Feb. 6, 1934 

1. A METHOD OF IMPROVING THE RESISTANCE TO CORROSION OF AN ALUMINUM ARTICLE HAVING A POROUS CHROMIUM ELECTROPLATE COMPRISING MAKING THE ARTICLE ANODE, AT A POTENTIAL OF AT LEAST 5 VOLTS, IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF WATER AND UP TO 1 GRAM PER LITER OF CHROMIC ACID, FOR A PERIOD OF AT LEAST 1/4 OF A MINUTE. 